The invention relates to recovery of oil and gas from wells, and more particularly to breaking fluids inside formation pores when using viscoelastic surfactant fluid systems (VES's) as carrier fluids and treatment fluids.
There are many applications in which breakers are needed to decrease the viscosity of treatment fluids, such as fracturing, gravel packing, and acidizing fluids, viscosified with polymers or crosslinked polymers or viscoelastic surfactants. Most commonly, these breakers act in fluids that are in gravel packs or fractures; some breakers can work in fluids in formation pores. Breakers decrease viscosity by degrading polymers or crosslinks when the viscosifiers are polymers or crosslinked polymers. Breakers decrease viscosity by degrading surfactants or destroying micelles when viscosifiers are viscoelastic surfactant fluid systems. Breakers can be solids, for example granules or encapsulated materials, that do not enter the formation.
There is sometimes a need to break viscous fluids within the pores of formations, for example when viscous fluids enter formations during fracturing, gravel packing, acidizing, matrix dissolution, lost circulation treatments, scale squeezes, and the like. Breakers that are effective inside formations will be called internal breakers here. These fluids that enter the formation may be main treatment fluids (such as fracturing fluids) or they may be secondary fluids (such as flushes or diversion fluids such as viscoelastic diverting acids). Typically it is necessary that the break be delayed, that is that the breaker not act until after the fluid has performed its function.
The current practice to improve clean-up of VES fluids in matrices is to use pre-flush or post-flush fluids to dilute the system or to contact the system with a breaker. The major disadvantage of the use pre-flush or post-flush fluids is their limited interaction with the VES fluid due to the small interface between the two fluids. The efficiency of this breaking mechanism depends upon diffusion, which is slow in highly viscous fluids. Furthermore, the volumes of the flushes can be high.
Compositions and treatment methods using a delayed internal breaker, that acts without mechanical or chemical action by the operator, would be of value. It would be desirable to have a number of such materials so that they could be used under different subterranean conditions, for example different temperatures and different formation fluid chemistries.
It has now been discovered that certain acids or the combinations of certain salt(s) and acid(s) will perform as internal breakers and will allow fluid design with pre-selectable timing for breaking of the fluid.